Method of making low nitrogen alloys

ABSTRACT

AN IMPROVEMENT IN THE MANUFACTURE OF STEEL OF LOW NITROGEN CONTENT, WHICH COMPRISES THE STEPS OF: COMPACTING STEEL HAVING AN AVERAGE THICKNESS OF LESS THAN 0.25 INCH INTO A POROUS MASS; HEATING THE POROUS MASS AT A TEMPERATURE BETWEEN ABOUT 2000*F. AND THE MELTING POINT OF THE STEEL, IN A VACUUM AT A PRESSURE BELOW ONE MM. HG, FOR A PERIOD OF TIME SUFFICIENT TO REMOVE AT LEAST 70% OF THE NITROGEN THEREFORM; MELTING THE POROUS MASS IN A FURNACE HAVING A CONTROLLED ATMOSPHERE; AND CASTING THE MELT.

8,788,836 Ice Patented Jan. 29, 1974 3,788,836 METHOD OF MAKING LOW NITROGEN ALLOYS Francis L. Muscatell, Loudonville, N.Y., and Sundaresan Ramachandran, Natrona Heights, and Orville W. Reen, Lower Burrell, Pa., assignors to Allegheny Ludlum Industries, Inc., Pittsburgh, Pa. No Drawing. Continuation-impart of abandoned application Ser. No. 720,481, Apr. 11, 1968. This application Mar. 26, 1971, Ser. No. 128,531

Int. Cl. C21c 7/10 U.S. on. 75-49 10 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-impart of now abandoned copending application Ser. No. 720,481, filed Apr. 11, 1968.

The present invention relates to an improvement in the manufacture of steel of low nitrogen content and more particularly to an improvement in the manufacture of steel of low nitrogen content wherein nitrogen is removed from solid steel having a high nitrogen content,

in an amount of at least 70% of the original content thereof.

In many steels the mechanical, physical, corrosion and/ or magnetic properties are dependent upon the amount of nitrogen present therein. For example, in stainless steels and other alloy steels in which corrosion resistance is particularly important, control of the nitrogen content is especially significant, as corrosion resistance is generally greater at lower nitrogen levels.

The melting of low nitrogen steel has typically been performed in furnaces having controlled atmospheres; e.g., inert gas or vacuum, in order to prevent the pickup of nitrogen by the molten steel and, if possible, to remove some nitrogen. While nitrogen pickup may be relatively easily prevented, the removal of nitrogen is more difiicult. Hence, low nitrogen steels had to be produced from raw materials of similar low nitrogen content.

A need for a process of producing steel from raw materials of high nitrogen content; e.g., scrap steel, existed prior to the present invention, since raw materials with low nitrogen contents are considerably more expensive than those with high nitrogen contents. Attempts were therefore made to remove nitrogen from commercial melts of high nitrogen content and from large solid masses of raw materials, but they were unsuccessful as shown in the examples reproduced hereinafter.

The present invention teaches an improvement in the manufacture of steel of low nitrogen content. More specifically, it teaches an improvement in the manufacture of steel of low nitrogen content wherein nitrogen is removed from solid steel having a high nitrogen content, in an amount of at least 70% of the original content thereof.

It is accordingly an object of the present invention to provide an improvement in the manufacture of steel of low nitrogen content.

The method of the present invention comprises the steps of: compacting steel having an average thickness of less than 0.25 inch, preferably less than 0.065 inch, into a porous mass; heating the porous mass at a temperature between about 2000 F. and the melting point of the steel, preferably at a temperature between 2200 F. and 2500 F., in a vacuum at a pressure below one mm. Hg, preferably below 10 mm. Hg, for a period of time, generally at least one hour, to remove at least 70%, preferably at least of the nitrogen therefrom; melting the porous mass in a furnace having a controlled atmosphere, generally a vacuum or inert gas; and casting the melt.

The degree of nitrogen removal is dependent upon temperature, time at temperature, degree of vacuum and size of the compacted steel. Nitrogen removal proceeds faster at higher temperatures and at lower pressures. For this reason, temperatures of at least 2200 F. and pressures below 10- mm. Hg are preferred, although temperatures as low as about 2000 F. and pressures as high as one mm. Hg are within the scope of the invention. Time at temperatures is generally at least one hour. Nitrogen removal is additionally dependent upon the thickness of the steel which forms the porous mass. Steel having an average thickness in excess of 0.25 inch is too large and has too small a surface area to permit adequate nitrogen removal. In connection with this, temperatures are generally kept below 2500 F. to preclude any possible fusion of the compacted steel and accompanying decrease in surface area.

The steel has an average nitrogen content of at least 150 p.p.m., generally at least 250 p.p.m., prior to nitrogen removal, and is quite often present in amounts of at least 1000 lbs. and generally in amounts of at least one ton, and can take various object forms such as machine tool turnings, edge trim and powder. Although it is preferable to compact the steel into one porous mass, it can be compacted into several. The final nitrogen content is generally below 50 p.p.m. and preferably below 30.

The following examples are illustrative of the invention. They are directed to stainless steel embodiments despite the fact that the invention is believed to be adaptable to other alloy steels and carbon steels, as stainless steels probably constitute its most important use.

EXAMPLE I Stainless steel chips having a nitrogen content of 250 p.p.m. and an average thickness of less than 0.25 inch were obtained by machining, and subsequently degreased. The composition of the chips was as follows, in wt. percent: 0.013% C, 1.67% Mn, 0.011%=P, 0.022% S, 0.41% Si, 18.74% Cr, 9.67% Ni, 0.025% N and balance essentially Fe.

The machined chips were compacted into a porous mass using a 400 ton load and subsequently denitrided. Denitridation involved the steps of transferring the porous mass into a vacuum furnace which was pumped down to the best of its capabilities, and heating the mass at a temperature of 2400 F. for 2 hours.

After denitriding, the steel had a nitrogen content of 45 p.p.m.; a decrease from the original value of 250 p.p.m. which is in excess of 80%.

The denitrided steel was subsequently: melted under vacuum and brought to a temperature of 2800 R; analyzed; adjusted with vacuum grade additions of silicon, chromium and nickel; subjected to higher pressures as the vacuum furnace was back filled with argon to mm. Hg; adjusted with carbon; and cast.

EXAMPLE II Steel plates having a thickness of 0.25 and the same composition as that of Example I were gradually melted under the vacuum of Example I and subsequently cast.

After casting, the nitrogen content was as high as 197 p.p.m.; a decrease from the original value of 250 p.p.m. of only about 20%. Moreover, a decrease which indicates that substantial nitrogen removal is not present with the diffusional resistance imposed by the 0.25 inch thick plates.

EXAMPLE III Additional compacted chips from Example I were treated as were the plates of Example II, with the exception that the chips were melted at a faster rate. The chips were present in an amount suflicient for a heat of approximately 7 pounds. After meltdown the nitrogen content was 141 p.p.m. and after casting it was 45 p.p.m. Time from meltdown to casting was approximately 30 minutes.

The nitrogen content of this example was lower after meltdown than was the nitrogen content of Example II after casting, as the chips did not offer the same resistance to difiusion as did the plates. Furthermore, the high loss in nitrogen from meltdown to casting for this example is attributable to the small laboratory size of the heat. Larger melts offer considerably more resistance to diffusion than do smaller heats as they are accompanied by a smaller melt surface to melt volume ratio, as is shown in the following example.

EXAMPLE IV Two 12,000 pound melts of stainless steel having a nitrogen content of 290 p.p.m. were vacuum refined at different pressures of 8x10" mm. Hg and 10" mm. Hg. The composition of the heats was as follows, in wt percent: 0.035% C., 0.66% Mn, 0.013% P, 0.020% S, 0.36% Si, 18.46% Cr, 8.67% Ni, 0.029% N and balance essentially Fe.

The first heat was maintained at 8X10 mm. Hg for approximately 50 minutes while the second heat was maintained at 10- mm. Hg for approximately 34 minutes. Neither heat had a nitrogen content much below 250 p.p.m. after treatment.

A comparison of the results of this example with the results of Example III clearly shows that larger melts offer considerably more resistance to diffusion than do smaller melts. For example, the nitrogen level of Example III dropped over 60% between meltdown and casting whereas it was less than in this example, despite the fact that the original melt nitrogen level was considerably higher in this example than in Example III.

EXAMPLE V Solid stainless steel having a nitrogen content of 340 p.p.m. and a thickness of 0.25 inch was denitrided in a vacuum of from 3 to 10 10- mm. Hg at a temperature of 2300" F. for 12 hours. The composition of the steel is as follows, in wt. percent: 0.053% C, 1,80% Mn, 0.017% P, 0.012% S, 0.46% Si, 18.59% Cr, 8.74% Ni, 0.034% N and balance essentially iron.

After denitriding, the nitrogen content of the steel was 196 p.p.m. on the outer surface and 274 p.p.m. in the center; a decrease from the original value of 340 p.p.m. which is less than 50% of the original value at the surface and less than 20% in the center. Moreover, a decrease does not compare favorably with Example I.

From the above paragraphs it will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they should not be limited to the specific examples described herein.

We claim:

1. A process of removing nitrogen form solid steel which comprises the steps of: compacting steel having an average thickness of less than 0.25 inch and an average nitrogen content of at least 150 p.p.m. into a porous mass; heating said porous mass at a temperature between about 2000 F. and the melting point of said steel, in a vacuum at a pressure below one mm. Hg, for a period of time sufiicient to remove at least of the nitrogen from said solid steel, said period of time being at least one hour; subsequently melting said porous mass in a furnace having a controlled atmosphere; and casting the melt.

2. An improvement according to claim 1 wherein said steel has an average thickness of less than 0.065 inch.

3. An improvement according to claim 1 wherein said porous mass is heated at a temperature between about 2200 F. and 2500 F., in said vacuum, for a period of time sufficient to remove at least 70% of the nitrogen therefrom.

4. An improvement according to claim 1 wherein said porous mass is heated at a temperature between about 2000 F. and the melting point of said steel, in a vacuum at a pressure below 10- mm. Hg.

5. An improvement according to claim 1 wherein at least 80% of the nitrogen is removed from said porous mass.

6. An improvement according to claim 1 wherein said initial nitrogen content is at least 250 p.p.m.

7. An improvement according to claim 1 wherein said steel is stainless steel.

8. An improvement according to claim 1 wherein said cast steel has a nitrogen content below 50 p.p.m.

9. An improvement according to claim 8 wherein said cast steel has a nitrogen content below 30 p.p.m.

10. An improvement according to claim 1 wherein said steel having an average thickness of less than 0.25 inch is present in an amount of at least 1,000 lbs.

References Cited UNITED STATES PATENTS 2,960,331 11/1960 Hanks -49 X 1,201,633 10/1916 Ruder 7549 X 1,555,314 9/1925 Rohn 75-49 2,036,496 4/1936 Randolph 75-49 X 2,997,760 8/ 1961 Hanks et a1. 75-49 X 3,222,161 12/1965 Luce et al 75-49 3,501,292 3/ 1970 Hart 75-49 X OTHER REFERENCES Child at al.: Article in Blast Furnace and Steel Plant. March 1959, pp. 281-289.

Gunow: Article in Vacuum Metallurgy, edited by Bunshah, Reinhold Publishing Co., New York, 1958, pp. 276, 282, 283.

L. DEWAYNE RUTLEDGE, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 

